mm: memory: remove ->vm_file check on shared writable vmas

Shared anonymous mmaps are implemented with shmem files, so all VMAs with
shared writable semantics also have an underlying backing file.
Signed-off-by: NJohannes Weiner <hannes@cmpxchg.org>
Reviewed-by: NJan Kara <jack@suse.cz>
Acked-by: NKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -2127,9 +2127,7 @@ static int do_wp_page(struct mm_struct *mm, struct vm_area_struct *vma,
 				balance_dirty_pages_ratelimited(mapping);
 			}
 
-			/* file_update_time outside page_lock */
-			if (vma->vm_file)
-				file_update_time(vma->vm_file);
+			file_update_time(vma->vm_file);
 		}
 		put_page(dirty_page);
 		if (page_mkwrite) {
@@ -2971,8 +2969,7 @@ static int do_shared_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 		balance_dirty_pages_ratelimited(mapping);
 	}
 
-	/* file_update_time outside page_lock */
-	if (vma->vm_file && !vma->vm_ops->page_mkwrite)
+	if (!vma->vm_ops->page_mkwrite)
 		file_update_time(vma->vm_file);
 
 	return ret;